This proposal is designed to develop new surgical procedures that will be safe and effective for the treatment of various types of supraventricular and ventricular arrhythmias. We propose the development and study of two canine models to produce atrial and ventricular arrhythmias. The first model would create chronic mitral regurgitation and left atrial enlargement by sectioning of the mitral valve apparatus with an extra-pericardial approach. Following long term recovery, spontaneous and pacing-induced atrial arrhythmias will be studied in anestetized open-chested animals using 260 bipolar electrodes constructed in multiple contoured templates. Real time computer construction of isochronous, isopotential, and refractory period maps will guide attempts at surgical ablation of these arrhythmias. The second model is designed to simulate myocardial ischemia as encountered following cardiac surgery that results in mild myocardial dysfunction and acute atrial and ventricular arrhythmias. Anoxic cardiac arrest utlizing cold potassium cardioplegic myocardial preservation will be accomplished. Regional myocardial cooling will be assessed by a new microsphere method. Ventricular electrophysiology will be assessed with multiple intramural plunge electrodes. Electrophysiologic atrial mapping will be recorded as above. Short term recovery with Holter monitoring, followed by repeated complete electrophysiologic analysis is planned. In both models a full understanding of the basic mechanisms of arrhythmias is the major goal with utilization of these mechanisms to direct the development of new surgical approaches to these problems.